The present invention relates to printers capable of operating in a unidirectional mode and in particular to a method of reducing the total printing pass time of ink-jet printers and plotters by appropriate co-ordination of the paper axis and scan axis movements.
Although movements of the print medium in the print medium (or paper) axis and movements of the printing carriage in the carriage (or scan) axis typically take only tenths of seconds, these non-printing periods add up to several seconds along a whole plot. Accordingly there is a need to optimise carriage and paper movements to decrease printing pass time and thus to increase the throughput of the printer.
In prior art printers operating in a bi-directional mode, such as those in the Hewlett-Packard DesignJet series, scan and paper axis movements during non-printing periods (i.e. when ink is not actually being applied to the paper) are performed simultaneously. This has the advantage of minimising the time between printing periods to a value equal to the greater of the duration of the scan axis movement and the duration of the paper axis movement.
Such a solution is not always achievable with unidirectional mode printers in which it may not be desirable or possible to make an advance along the paper axis while a scan axis movement of maximum return speed is being undertaken. For example, if both these movements occur simultaneously, the paper may lift off the printer platen and come into contact with the printhead in an undesired manner. In addition, more complex movement algorithms would be required to control the servos of the carriage and paper movement motors in parallel; this would require more CPU processing time which would interrupt or delay the preparation of data for the next printing pass. Moreover, a higher power consumption would be required in particular higher current peaks which would involve more expensive power components and would tend to produce more electrical interference. Accordingly, the present invention seeks to overcome the problem of optimising total printing time in such unidirectional mode printers. It seeks to achieve this by specifying an appropriate algorithm.
According to a first aspect of the present invention, there is provided a method of operating a printer in a unidirectional mode in which, at the end of a printing pass, a printing carriage is:
(i) in a first phase, decelerated from its printing speed and accelerated in the opposite direction to a return speed;
(ii) in a second phase, returned towards its start end at said return speed; and
(iii) in a third phase, decelerated to zero speed and accelerated in the printing direction to its printing speed;
and during the above period an appropriate advance occurs in the print medium axis so that the next printing pass can start,
characterised in that print medium advance movements are undertaken in both the first and third phases.
An advantage of the above method is that time is not wasted in unnecessarily performing scan axis movements and paper axis movements sequentially. Instead one avoids as much as possible a decelerating or accelerating scan axis movement at either end of the scan axis without an accompanying print medium axis movement.
If the print medium advance takes longer than the sum of the decelerating and accelerating scan axis movements, it is completed adjacent in time to these movements at only one end of the scan axis (i.e. directly preceding and/or directly following these movements). This ensures that the print medium advance occurs as quickly as possible, thus contributing to the improvement of throughput.
According to a second aspect of the present invention, there is provided a printer capable of operating in a unidirectional mode and comprising a printing carriage, carriage movement means for causing the carriage to move through a printing pass, decelerating the carriage at the end of a printing pass, accelerating the carriage in the opposite direction, returning the carriage towards its start end, at a substantially constant speed, decelerating the carriage at its start end and accelerating the carriage to its printing speed, and print medium advance means for advancing a print medium between printing passes, characterised in that the arrangement is such that the advance means moves the print medium during two periods separated in time by the period during which the carriage movement means is returning the carriage towards its start end at the substantially constant speed.